This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
As the global demand for energy has increased, alternative energy generation systems have become an increasingly important component in the worldwide energy generation scheme. Similarly, as the focus on air pollution and energy generation emissions has increased, much attention has focused on fuel cells as a clean and portable source of energy. A primary hindrance to widespread commercial use, however, is that fuel cells require a costly platinum (Pt) catalyst for operation. Therefore, reducing the amount of Pt required will increase the economic viability of fuel cells.
The foreground of sustainable energy is built on a renewable and environment-compatible scheme of chemical-electrical energy conversion. One of the key processes for such energy conversion is the electrocatalytic reduction of oxygen, which is the cathode reaction in fuel cells and metal-air batteries where an electrocatalyst is used to accelerate the course of ORR. Current electrocatalysts used for this reaction are typically in the form of dispersed Pt nanoparticles on amorphous high-surface-area carbon. Considering the high cost and limited availability of Pt, large-scale applications of these renewable energy technologies demand substantial improvement of the catalyst performance so that the amount of Pt needed can be significantly reduced. For example, a five-fold improvement of catalytic activity for the ORR is required for the commercial implementation of fuel cell technology in transportation.
The advancement of heterogeneous catalysis relies on the capability of altering material structures at the nanoscale. Such alteration is particularly important for the development of highly active electrocatalysts with uncompromised durability. This disclosure reports the design and synthesis of a Pt-bimetallic catalyst with multilayered Pt-skin surface that provides superior electrocatalytic performance for the oxygen reduction reaction (ORR). This novel structure was first established on extended thin film surfaces with tailored composition profiles and then implemented in nanocatalysts by organic solution synthesis. Electrochemical studies for the ORR demonstrated that, after elongated exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surfaces exhibits an improvement factor in activity of more than one order of magnitude versus conventional Pt catalysts. This substantially enhanced catalytic activity, as well as improved durability, indicate great potential toward improving the material properties by fine tuning of the nanoscale architecture.
Prior work on well-defined extended surfaces has shown that high catalytic activity for the ORR can be achieved on Pt-bimetallic alloys (Pt3M, M=Fe, Co, Ni, etc.), due to the altered electronic structure of the Pt topmost layer and hence reduced adsorption of oxygenated spectator species (e.g., OH−) on the surface. It was also found that in acidic electrochemical environment the non-noble 3d transition metals are dissolved from the near-surface layers, which leads to the formation of a Pt-skeleton surface. Moreover, the thermal treatment of Pt3M alloys in ultra high vacuum (UHV) has been shown to induce segregation of Pt and formation of distinctive topmost layer that was termed Pt-skin surface. However, the same treatment did not cause Pt to segregate over PtM alloys with high content (≦50%) of non-Pt elements. More recently, the surfacing of an ordered Pt(111)-skin over Pt3Ni(111) single crystal having 50% of Ni in the subsurface layer was further demonstrated. This unique nanosegregated composition profile was found to be responsible for the dramatically enhanced ORR activity.
Based on these findings, it could be envisioned that the most advantageous nanoscale architecture for a bimetallic electrocatalyst would correspond to the segregated Pt-skin composition profile established on extended surfaces. Much effort has been dedicated, but it still remains elusive, to finely tune the Pt-bimetallic nanostructure in order to achieve this desirable surface structure and composition profile. Major obstacles reside not only in the difficulty for manipulation of elemental distribution at the nanoscale, but also in the fundamental differences in atomic structures, electronic properties and catalytic performance between extended surfaces and confined nanomaterials. For example, in attempt to induce surface segregation, high-temperature (greater than 600 degrees Celsius) annealing is typically applied for Pt-based alloy nanocatalysts. While improvement in specific activity is obtained, such treatment usually causes particle sintering and loss of electrochemical surface area (ECSA). Besides that, the surface coordination of nanomaterials is quite different from that of bulk materials, i.e., the surface of nanoparticles is rich in corner and edge sites, which have smaller coordination number than the atoms on long range ordered terraces of extended surfaces. These low-coordination surface atoms are considered as preferential sites for the adsorption of oxygenated spectator species (e.g., OH−), and thus become blocked for adsorption of molecular oxygen and inactive for the ORR. Additionally, due to strong Pt—O interaction these low-coordination atoms are more vulnerable for migration and dissolution, resulting in poor durability and fast decay of the catalyst. The latter effect is even more pronounced in Pt-bimetallic systems, considering that more low-coordination sites are present on the skeleton surfaces formed after the depletion of non-precious metals from near-surface regions.